Butane cracking

ABSTRACT

A process for the preparation of ethylene and propylene wherein n-butane is cracked at high pressure in the presence of steam, preferably with ammonia in a reactor containing a solid support, at a temperature of from 550* to 750* C. for a residence time of 0.2 to 15 seconds to convert 40 to 90 percent of the n-butane. The reaction product is cooled and methane separated therefrom at a pressure not greater than that in the cracking reaction. The C4 fraction separated from the reaction product is recycled to the cracking step.

United States Patent [1 1 Espino et a].

[4 1 Sept. 25, 1973 BUTANE CRACKING 2,8l6,l 12/1957 Hepp 260/683 Inventors: Ramon L. Espino, New York, N,.Y.;

Martin B. Sherwin, Paramus; Gerald f EMmI'fEF-DebeH 5 Gang Sugel-man, parsippany both of Ni Assistant Examzner-C. E. Spresser Att0rneyBert J. Lewen [73] Assignee: Chem Systems, Inc., New York, NY.

[22] Filed: Feb. 11, l97l 57 ABSTRACT 1 1 pp N03 114,528 A process for the preparation of ethylene and propylene wherein n-butane is cracked at high pressure in the UISI Cl n A presence of steam, preferably With ammonia in a reac. [51 Int. Cl. .f. C07C 3/30 Containing a Solid Support a temperature Offmm 58 Field Of Search 2607683 677 A" 750C for a residence time 15 Seccmds 208/10] 100 103 to convert 40 to 90 percent of the n-butane. The reac- 7 tion product is cooled and methane separated there- [56] References Cied from at a pressure not greater than that in the cracking reaction. The C fraction separated from the reaction UNITED STATES PATENTS product is recycled to the cracking step. 1,939,084 12/1933 Rosenthal .Qr.. 260/683 2,240,433 4/1941 Atw'ell..... 208/101 10 Claims, l Drawing Figure 1 [nu/Y: Q-cyuc 5/264 7 fzww: z row Ce ts 02 CH 0 @5001 Cam /x50? D/w0v7\ H 4w! 6/754: Q [U fiLuE/W' 6 c; ,3 ,5

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Q ab newc Z2 GE 24 \k U E 2 fizm/vsflwzws Swat v 5* 1 BUTANE CRACKING This invention relates to the preparation of low molecular weight olefins by the cracking of normal butane, under selected conditions; and to an integrated process for the separation, purification and recycling of the reaction products. More specifically, the invention teaches a high pressure process for the preparation of unusually high yields of propylene and ethylene based on the normal butane feedstock.

In the prior art there are numerous references to the cracking of low molecular weight paraffins, such as nbutane, to produce olefins. These references primarily consider the reaction as an initial step in the preparation of high octane gasoline. Unfortunately, these prior art processes are not satisfactory for the economical preparation of propylene, a basic chemical now in great demand,,since they produce extremely large amounts of methane and other hydrocarbons of lesser value in today's market.

In accordance with this invention, it has now been discovered that high yields of ethylene and propylene, particularly the latter, can be obtained by selecting and carefully controlling the. temperature, pressure, and residence time of the reaction to achieve a n-butane conversion of between 40 and 90 percent. Additionally it is necessary to add a specified amount of a diluent, e.g., steam, to the reaction vessel.

Table A sets forth the broad and preferred ranges for the key operating conditions.

TABLE A Operating Condition Broad Preferred Temperature 550-750"C. GOO-700C Pressure 300-700 psig 550-650 psig Residence time 0.2-l5 sec. I-5 sec. Diluent weight 0.5- parts/part l 3 parts/part ratio hydrocarbon hydrocarbon By utilizing the aforesaid conditions high ultimate yields of propylene and ethylene may be obtained with or without a solid heterogenous catalyst. The reactor effluent obtained is readily fractionated without the need of substantial compression and contains a butanebutene blend which may be directly recycled to the reaction vessel.

Furthermore, and quite surprisingly, the n-butane rich stream is cracked without the production of a significant amount of heavy oil or coke. Previous attempts to crack such hydrocarbon streams at high pressure resulted in the formation of heavy oils and carbonaceous materials which in turn lessened the yield, made the process more difficult to operate, and quickly inactivated any catalyst employed.

Many desirable economic advantages are obtained by the inventive process. Butane is commerically transported, under pressure, in liquefied form. Prior processes depressurized the butane in the cracking step. High pressure operation eliminates the depressuring and, more importantly, avoids the need to compress prior to fractionation. High pressure cracking also suppresses the formation of accetylenic compounds which required selective hydrogenation for their removal. Lower cracking temperatures permit the use of lower cost reactor tube materials and the formation of a product richer in propylene.

It is not desirable to use temperatures over 750C., because this leads to undesirable hydrocarbon cracking resulting in the formation of unwanted methane and coke. Temperatures below 550C. make conversions too low for practical operation. Higher pressures than those set forth in Table A are disadvantageous because of excessive polymerization and back hydrogenation of the olefins as well as yield loss and coke formation. Lower pressures significantly decrease the conversion.

The residence time, i. e., the ratio of the actual volume occupied by the gas in the reactor to the volumetric inlet flow into the reactor at the pressure and temperature of the unit, set forth in Table A is particularly important with regard to the ultimate product distribution. Shorter residence times require higher operating temperatures, while longer times results in degradation to methane and coke.

It is imperative to maintain a conversion level from 40 to percent. Conversions above the upper range cause a sharp drop in selectivity to the desired products. There is an unnecessary loss of productivity at conversions below 40 percent.

The diluent set forth in Table A is most preferably steam or a mixture of steam and ammonia, where up to 40 percent of the latter is added. Preferably from 10 to 35 weight percent of ammonia is present. Use of a diluent enhances the yield and the overall operability of the process. Lower amounts than those set forth result in increased coking and heavy oilformation. Higher amounts are of no further benefit to the process and diminish the conversion rates and require higher temperature operation. The ammonia is particularly desirable, because it serves to diminish the amount of heavy hydrocarbons, that is, those above C, in weight, during reaction.

When a tubular reactor is employed, it is particularly beneficial to fill these tubes with a solid support which further acts to diminish the amount of coking and heavy oil formed. Examples of solid supports which may be used are non-acidic types, such as, kaolin and magnesium oxide, and blends of 70 percent or more kaolin. These supports have a surface area of from 1/2 to square meters per gram. Other types of reactors can be employed. Since the reaction is endothermic and heat must be added to the system, a tubular furnace where radiant heat is provided to the tube walls in the simplest. Alternatively, a tubular reactor immersed in a high temperature heat transfer agent, such as lead or a moving bed of pebbles, may be employed. Obviously, the type of reactor is not critical and numerous other types known to those skilled in the art are adaptable.

Conventional columns and conditions may be employed for the de-methanizer and de-ethanizer which are essential units in the integrated process. These factors are known to those skilled in the art and exemplifled in the process description set forth herein. It is the necessity of operating these units at high pressure which makes high pressure cracking particularly benefical in practicing the invention.

It is a particularly preferred embodiment of the subject invention to recycle all of the unreacted butane and butenes formed in the reaction directly back to the reactor. Only by following the process of the invention can this practice be feasibly employed. In the prior art, it was necessary to separate the butenes or hydrogenate them to butane before recycle was possible. Naturally these additional processing steps added to the expense of the overall process without any benefit whatever. This embodiment is feasible in the instant process because the butene concentration in the C recycle stream is less than 25 percent. The tolerance of the system for such amounts of butene is particularly surprising since the presence of such material would normally be expected to lead to significant coking and the loss of yields. Furthermore, there is no built-up of these materials in the reactor, possibly because they are hydrogenated therein under the unique high pressure operation.

In order to more fully illustrate the invention, attention is directed to the attached FIGURE:

Fresh butane feed, 1,210 parts, fed through line 1, 2,150 parts of steam diluent added via line 2, and a butane-butene stream containing 1,073 parts of butane and 155 parts of butenes recycled via line 3 are added to the butane reactor 4. The reactor 4 is maintained at a temperature of 670 C and a pressure of 550 psig. The diluent to hydrocarbon molar ratio is 3:1. After a residence time of 5 seconds, the reaction product is withdrawn from the reactor 4 via line 5 and fed to the quench vessel 6. The water is separated via line 7 and recycled (not shown). Hydrocarbons, 2,437 parts, are fed via line 8 to the de-methanizer 10, along with 321 parts of the compressed material in line 9 from the ethane cracker 18. The de-methanizer is operated at a pressure of 500 psig, an overhead temperature of -70 to 90 C a bottoms temperature of 10 to 25 C, and has 60 to 80 trays and a reflux ratio of 0.5 to 0.75. 252 parts of methane and hydrogen are removed overhead from the de-methanizer 10 via line 11. The bottoms, 2,506 parts, are fed to the de-ethanizer 13 via line 12.

The de-ethanizer is operated at a pressure of 395 psig, an overhead temperature of from 12 to 8 C, a bottoms temperature of 70 to 82 C, and has 40 to 60 trays and a reflux ratio of from 0.4 to 0.6. A mixture of 661 parts of ethylene and ethane are separated from the top of the de-ethanizer l3 and fed via line 14 to the ethylene-ethane splitter 15. The ethylene product, 340 parts, is removed from the process via line 16. Ethane, 321 parts, which may be either separated from the process or recycled, is removed from the splitter 15 via line 17. 1f recycled, the ethane is passed to the ethane cracker 18 along with steam. The reaction product from the ethane reactor 18 is fed via line 19 to quench vessel 20 and compressor 21 and the product passed to the de-ethanizer 10 as previously described.

The bottoms from the de-ethanizer 13, 1,845 parts, are passed via line 22 to the C /C splitter 23 wherein 527 parts of 95 percent pure propylene and 5 percent propane is removed overhead via line 24. This is the main product of the process.

The bottoms, 1,318 parts, from the C /C splitter 23 are sent via line 25 to the C /C splitter 26. High boiling products, 90 parts, are removed as a liquid via line 27. These contain hydrocarbons having five or more carbon atoms. The overhead, 1,228 parts, from the CJC, splitter 26 is removed via line 3 and recycled to the butane reactor 4 as previously described.

As can be readily seen from the above figure, for each 100 parts by weight of butane fed to the reaction 28 parts of ethylene and 41.5 parts of propylene are obtained. This gives a propylene to ethylene ratio of 1.5:1 at a percent conversion to n-butane of 50 percent.

In the event the butene in the stream 3 becomes greater than 25 percent, or it is desirable to completely eliminate the butene from the recycle, two alternate procedures may be readily employed. Firstly, the butenes may be separated from the butane by extraction or distillation; secondly, it may be hydrogenated back to butane by contact with hydrogen over a conventional hydrogenation catalyst, e.g., platinum, palladium or nickel on supports.

The following examples are set forth to further show the advantages of the instant invention:

EXAMPLE 1 The operation of the process at various temperatures and percent conversions is shown in Table 1. The reaction is carried out in a 1 inch X 8 foot reactor at a throughput of 1,500 STP liters/hours of total feed. The molar ratio of steam diluent to hydrocarbon is 2:1 and the total reaction pressure 550 psig.

TABLE 1 Run No. A B Temperature, "C 645 675 Residence time, Sec. 6.5 6.5 Conversion, 45 85 Yield to propylene,

ethylene and ethane: 71 61 Weight Ratio propylenelethylene 1.5 1.0

t This calculation is based on a conversion of each part of ethane to 0.8 parts of ethylene in an ethane cracker, such as shown in the figure.

This example shows that high yields and propylene ratios can be obtained over the range of conversions claimed.

EXAMPLE 11 Using the same reactor and diluent ratio as set forth in Example 1, the reaction was carried out at various pressures. The following results were obtained:

This calculation is based on a conversion of each part of ethane to 0.8 parts of ethylene in an ethane cracker, such as shown in the figure.

Runs D and E describe the practice of the instant invention. Clearly, in this range there is marked increase in the yields to the desired products, particularly to propylene.

EXAMPLE 111 The effect of ammonia and a solid support is shown in Table 3. The reaction was carried out in a 2 inch 1.D. X 4 inch reactor and the volumetric ratio of the diluent to hydrocarbon at the inlet was 2:1. The residence time in all these runs was 6 seconds at inlet feed conditions. The total pressure was 600 psig.

TABLE 3 Tube packed with Empty Tube Kaolin Pellets G H l .1

Weight NH, in diluent 0 30 0 30 Temperature, C 640 645 635 650 Conversion. 1: 68 63 70 Weight & Selectivity to:

Methane 18.0 19.0 18.4 20.0 Ethane 16.7 16.2 16.0 16.0 Ethylene 15.6 16.0 16.0 16.5 Propylene 34.0 33.0 34.0 33.0 Propane 4.0 3.2 3.6 3.5 C Olefins 4.2 5.5 6.0 6.5

These runs show that the use of 30 percent ammonia in the diluent and packing the reaction tube with a solid support, suchas kaolin, reduces the formation of high boiling byproducts. Run J shows that the use of these two preferred embodiments together is still better.

Having thus described our invention, what we claim and and desire to protect by Letters Patent is:

1. A process for the preparation of ethylene and propylene which comprises: cracking a feed containing predominantly n-butane and a diluent, the diluent being composed of from 0.5 to parts by weight of steam from each part of hydrocarbon and at least 10 percent of ammonia based on total diluent, in a reaction zone at a temperature of from 550 to 750 C., a pressure of about 550 to 700 psig, for a residence time of from 0.2 to seconds to convert 40 to 90 percent of said n-butane; withdrawing a reaction product from said reaction zone which contains methane, a C, fraction and a fraction which is rich in ethylene and propylene; cooling said reaction product and separating the methane therefrom in a distillation zone at a pressure not greater than the pressure in the reaction zone; removing the ethylene, propylene and C, fraction from said distillation zone; and thereafter separating the ethylene and propylene as products.

2. The process of claim 1 wherein the temperature is from 600 to 700 C., the pressure of from 550 to 650 psig, the residence time from 1 to 5 seconds and the diluent level of from one to three parts by weight for each part of hydrocarbon.

3. The process of claim 1 wherein the diluent contains up to 40 percent by weight of ammonia.

4. The process of claim 1 wherein said reaction is carried out in the presence of a nonacidic low to moder ate surface area support.

5. The process of claim 1 wherein said support is kaolin or magnesium oxide.

6. The process of claim 1 wherein the C, fraction containing n-butane and butene is separated from the reaction product and recycled directly to the cracking step.

7. A process for the preparation of ethylene and propylene which comprises: cracking a feed containing predominantly n-butane and from 0.5 to 10 parts by weight of steam for each part of hydrocarbon in a reaction zone at a temperature of from 550 to 750 C., a pressure of about 550 to 700 psig, for a residence time of from 0.2 to 15 seconds to convert 40 to percent of said n-butane; withdrawing a reaction product from said reaction zone which contains methane, 'a C, fraction and a fraction which is rich in ethylene and propylene; cooling said reaction product and passing it directly to a distillation zone, separating the methane from said reaction product in said distillation zone at a pressure of about 500 psig; removing the ethylene, propylene and C, fraction from said distillation zone; and thereafter separating the ethylene and propylene as products.

8. The process of claim 7 wherein the temperature is from 600 to 700 C., the pressure from 550 to 650 psig, the residence time from 1 to 5 seconds, and wherein the feed contains from one to three parts by weight of steam for each part of hydrocarbon.

9. The process of claim 7 wherein said reaction is carried out in the presence of a non-acidic low to moderate surface area support.

10. The process of claim 7 wherein the C, fraction containing n-butane and butene is separated from the reaction product and recycled directly to the cracking step. 

2. The process of claim 1 wherein the temperature is from 600* to 700* C., the pressure of from 550 to 650 psig, the residence time from 1 to 5 seconds and the diluent level of from one to three parts by weight for each part of hydrocarbon.
 3. The process of claim 1 wherein the diluent contains up to 40 percent by weight of ammonia.
 4. The process of claim 1 wherein said reaction is carried out in the presence of a non-acidic low to moderate surface area support.
 5. The process of claim 1 wherein said support is kaolin or magnesium oxide.
 6. The process of claim 1 wherein the C4 fraction containing n-butane and butene is separated from the reaction product and recycled directly to the cracking step.
 7. A process for the preparation of ethylene and propylene which comprises: cracking a feed containing predominantly n-butane and from 0.5 to 10 parts by weight of steam for each part of hydrocarbon in a reaction zone at a temperature of from 550* to 750* C., a pressure of about 550 to 700 psig, for a residence time of from 0.2 to 15 seconds to convert 40 to 90 percent of said n-butane; withdrawing a reaction product from said reaction zone which contains methane, a C4 fraction and a fraction which is rich in ethylene and propylene; cooling said reaction product and passing it directly to a distillation zone, separating the methane from said reaction product in said distillation zone at a pressure of about 500 psig; removing the ethylene, propylene and C4 fraction from said distillation zone; and thereafter separating the ethylene and propylene as products.
 8. The process of claim 7 wherein the temperature is from 600* to 700* C., the pressure from 550 to 650 psig, the residence time from 1 to 5 seconds, and wherein the feed contains from one to three parts by weight of steam for each part of hydrocarbon.
 9. The process of claim 7 wherein said reaction is carried out in the presence of a non-acidic low to moderate surface area support.
 10. The process of claim 7 wherein the C4 fraction containing n-butane and butene is separated from the reaction product and recycled directly to the cracking step. 